The present invention relates to a method of presuming the life time of a semiconductor device, and particularly, to improvement in an evaluation method.
In recent years, as semiconductors have smaller and smaller geometry, reliability of a semiconductor device is becoming more important. For example, reliability of a gate oxide film of a MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor) determines characteristics of a semiconductor device itself, and therefore, a number of achievements of research have been disclosed. If a transistor has a gate length of approximately 0.35 .mu.m, the thickness of a gate oxide film is 8 to 9 nm. Hence, to form the gate oxide film with an excellent characteristic and to reduce damage to the gate oxide film due to plasma or the like during processes bear an increasing technical importance.
It is necessary to find the level of damage to gate oxide films during processes to reduce deterioration of the gate oxide films. To grasp the level of the damage, it is necessary to accurately know the life time of each one of the gate oxide films which are damaged differently. To satisfy this need, presumption of the life time of a gate oxide film is also becoming important. The following evaluation methods are currently known as a method to presume the life time:
1) Evaluation utilizing an applied lamp voltage; PA1 2) Evaluation utilizing an applied constant voltage; and PA1 3) Evaluation utilizing an implanted constant current.
FIGS. 17a to 17c are flow charts of the three evaluation methods above.
As shown in FIG. 17a, the evaluation method 1) utilizing an applied lamp voltage evaluates a wafer as a whole. At Step ST1, a lamp voltage is applied to an insulation film which is formed on the wafer and the applied voltage is increased. At Step ST2, a current value at the time when insulation breakdown occurs in the wafer and a current flows is monitored. At Step ST3 and Step ST4, breakdown modes are classified and whether Good or Bad is judged. Breakdown occurs in about 10.sup.3 sec.
As shown in FIG. 17b, in the evaluation method 2) utilizing an applied constant voltage, a constant voltage is applied to an insulation film at Step ST11 and a current value is monitored during application of the constant voltage at Step ST12. The applied constant voltage alters the insulation film with time. As breakdown of the insulation film occurs due to deterioration, the time at which breakdown of the insulation film occurs is detected at Step ST13. Breakdown occurs in about 10.sup.5 sec.
As shown in FIG. 17c, in the evaluation method 3) utilizing an implanted constant current, a constant current is implanted into an insulation film at Step ST21. At Step ST22, a voltage value which increases with time is monitored. The time at which breakdown of the insulation film occurs is detected at Step ST23. Breakdown occurs in about 10.sup.4 sec.
Further, a TFT (Thin-Film-Transistor) as well which is used in a liquid crystal display panel, a demand for which has been noticeably growing recently in particular, must comprise a reliable gate oxide film. The reliability is evaluated basically by the evaluation methods described above.
In addition, a number of engineers are involved in developing improved evaluation methods of evaluating reliability of elements other than that of a gate oxide film. For instance, metal wires used in a semiconductor device deteriorate with time and electrically fracture due to a defect known as a void (i.e., electro migration, hereinafter "EM"). A widely used evaluation method is a method in which a constant current implanted into a metal wire and a time until breakdown is measured while monitoring a resistance value of the metal wire. This method as well, presuming the life time based on a current value, may not accurately presume the life time of the metal wire used in a semiconductor device which operates basically under the control of a voltage. While some evaluation methods use a constant voltage, such methods take a longer time for evaluation than methods which use an implanted constant current, which situation is similar to evaluation of gate oxide films.
Table 1 shows advantages and disadvantages of the three evaluation methods 1) to 3) such as a time needed for evaluation and the accuracy of life time presumption.
TABLE 1 ______________________________________ Measurement Time Quantification ______________________________________ Applied Lamp -10.sup.3 s Inappropriate Voltage Applied Constant &gt;10.sup.5 s Appropriate Voltage Implanted Constant -10.sup.4 s Most Appropriate Current ______________________________________
From Table 1, it is understood that these evaluation methods have the following problems.
The evaluation method 1) utilizing an applied lamp voltage takes the shortest time to evaluate among the evaluation methods. However, since the method 1) detects only whether breakdown has occurred, it is difficult to quantify deterioration of a gate oxide film or to presume the life time of the gate oxide film, although comparative life time measurement is possible.
Next, the evaluation method 2) utilizing an applied constant voltage is the most popular as a life time presumption method. This is because semiconductor devices are generally voltage-controlled and therefore the life time presumed by the evaluation method 2) is a very accurate value which well expresses the actual condition of the semiconductor devices. However, a time necessary for evaluation is the longest, 10.sup.5 sec among the three evaluation methods, requiring rather many days for evaluation. While the evaluation method 2) is applicable to a so-called accelerated test which is conducted with a high substrate temperature, if a testing time is to be reduced to a practical level, the accelerated test must be conducted under a substantially high temperature and hence errors in data become extremely large or other problems occurs. For this reason, the evaluation method 2) is not likely to yield quantitative, fast and accurate evaluation.
On the other hand, a time needed for evaluation in the evaluation method 3) utilizing an implanted constant current is about 1/10 or shorter than that in the evaluation method 2). It is possible to quantify deterioration of a gate oxide film in the evaluation method 3). However, since a resistance of the insulation film increases because electric charges are trapped as a current is implanted, a voltage value needs be increased to ensure that a current of a constant value flows. Despite this need, since a semiconductor device is controlled by a constant voltage in reality, the life time presumed by changing the voltage and flowing a constant current is not an enough accurate value which well expresses the actual condition of the gate oxide film.
Evaluation of EM in a metal wire described earlier is similar to evaluation of an insulation film in that the conventional evaluation utilizing an implanted constant current cannot realize accurate life time presumption.
In addition to an unpractically long testing time, in some cases, a test itself is difficult under a certain electric stress.